1. Field of the Invention
The present invention relates to a microbridge structure in which a thin film section spaced from a substrate is supported by a supporting section.
2. Description of the Related Art
As an example of devices which use a microbridge structure, a non-cooling type infrared image microbridge sensor is known.
FIG. 1 is a diagram illustrating a sensor element of a conventional non-cooling type infrared ray image sensor which is manufactured using an IC manufacturing technique. The sensor element is composed of a sensor section 2 and supporting sections 3 and 4. FIG. 2 is a perspective view which shows the shape of a part of the supporting sections 3 and 4 of the conventional sensor element.
In the infrared image sensor, a plurality of sensor elements typically arranged in a 2-dimensional matrix. Each sensor element is formed as shown in FIG. 1. The sensor element is of a bolometer type in which the change of a resistance value of the sensor section 2 caused by heating. of infrared rays which are incident to the sensor section 2 is taken out as an electric signal. In order to decrease the sensor sensitivity, the sensor section 2 is formed to have a microbridge structure in which the sensor section 2 is spaced from a substrate 1.
Also, a sensor resistance pattern 5 which is composed of material whose resistance value changes depending upon temperature is formed on the surface of the sensor section 2. A pattern 8 which is composed of the same material as that of the sensor resistance pattern 5 is formed on each of the supporting sections 3 and 4. The detecting result of the sensor section 2 is electrically connected to an output circuit (not illustrated) which is formed on the substrate 1, as a sensor signal through the supporting sections 3 and 4. At least one of the following matters must be implemented to improve the sensor sensitivity. That is, (1) a quantity of infrared rays which are absorbed by the sensor section 2 is increased, (2) noise which is generated in the sensor section 2 is reduced, and (3) heat insulation of the sensor section 2 is improved. In order to improve a responsivity in addition to the sensor sensitivity, it is necessary to make thermal capacity of the sensor section 2 small.
In order to decrease thermal capacity of the sensor section 2, it could be considered that material with a small specific heat is used for the sensor section 2, that the film thickness of the sensor section 2 is made thin, and that the area of the sensor section 2 is made small. However, silicon oxide or silicon nitride is generally used for the sensor section 2 for the reason of the manufacturing process wed to form same. Thus, the materials which may be used to form to the sensor section 2 are limited as a practical matter. Also, the area of the sensor section 2 cannot be made small sometimes. In order for the incident infrared rays to be absorbed sufficiently, there is a case that the area of the sensor section 2 is made large.
In the such case, in order to decrease the thermal capacity of the sensor section 2, the sensor section 2 must be made thin. However, when the sensor section 2 is spaced from the substrate 1, a problem lies in making the film thickness of the sensor section 2 thin from the viewpoint of strength of the sensor section 2.
Paying attention to the supporting sections 3 and 4, it is necessary that a material with low thermal conductivity is used in forming same and the supporting sections 3 and 4 can be made thin, narrow and long, to accomplish heat isolation of the sensor section 2. However, materials which can be used in the micromachining process are limited and the supporting sections 3 and 4 have materials used thereof and strength thereof.
In this way, in the infrared sensor element adopting the bridge structure in which the sensor section 2 is spaced from the substrate 1 by the two supporting sections 3 and 4, the heat isolation and thermal capacity of the sensor section 2 have a trade off relation with the strength of the sensor section 2 in structure.